Oxide coatings deposited in Glenn Research Center's Plasma Spray-Physical Vapor Deposition (PS-PVD) facility can be processed to be mechanically tough (erosion-resistant) and electrically conductive at room temperature. The electrically conductive phase contained within the oxide (MO2) coatings is a metastable suboxide (MO) that has not been formed in significant volume under any other known methods. Content of the electrically conductive phase can be varied in addition to the microstructure, which can be columnar, planar, or a combination of the two depending on the processing conditions. Upon exposing the material to moderate temperatures (>300 °C), the metastable phase is further oxidized (to MO2) and the material becomes insulating, but retains its high toughness and microstructure.

This invention is a coating deposited under a set of conditions using PS-PVD. Analysis of coatings using X-ray diffraction indicates the coatings are a mixture of a number of oxidation states. For example, coatings of zirconia have been shown to contain ZrO2, ZrO, and Zr3O. The coatings themselves have shown some electrical conductivity using point probes, and this is directly related to the presence of the metastable suboxide states (ZrO, Zr3O).

The coatings deposited via the liquid or vapor phase are columnar, planar, or a combination of the two geometries. By using the thermal plasma generated by the PS-PVD torch, feedstock is injected and heated to temperatures high enough to melt or vaporize the ceramic material. The resultant coatings contain the metastable reduced oxide phase, and will retain these properties unless they are heated in an oxygen-containing environment.

It was found that the coating volume of this metastable phase could be controlled by changing the processing parameters. Additional materials also displayed some of the same traits when deposited under certain conditions with PS-PVD, as HfO was observed while depositing HfO2, and YbO was observed while depositing Yb2Si2O7. The temperature and oxidation sensitivity were tested, and it was found that the phases dissipated upon heating above ~300 °C in oxygen-containing environments.

The unique aspect of this process is that the phases formed are not usually found outside of the vapor state. If ZrO2 or HfO2 (the thermodynamically stable phases) were reduced using a standard process reduction at high temperature, they oxidize from the dioxide to the metallic phase (i.e., ZrO2 changes to Zr metal); however, with the PS-PVD process conditions, the metastable oxide can be deposited. The volume content of this metastable oxide and its microstructure can be varied depending on the processing parameters as well. Initial modeling has suggested that the ZrO material is conductive, and probe measurements have confirmed very low resistance or fully conductive coatings. However, because this material is metastable, it can be easily annealed out in the presence of oxygen at temperatures above 300 °C.

Although coatings are oxide in nature, the prevalence of ZrO in the coating provides some conductivity. The crystallo-graphic domains display some amount of texture (crystal orientation) and very small domains (on the order of 20-30nm). Due to the PS-PVD process, the coatings can be applied onto complex shapes via some non-line-of-sight deposition. Coatings can be deposited via vapor or liquid methods to form dense layers or columnar structures with texture.

This work was done by Bryan Harder, Michael P. Schmitt, and Brian S. Good of Glenn Research Center. NASA is seeking partners to further develop this technology through joint cooperative research and development. For more information about this technology and to explore opportunities, please contact here . LEW-19385-1

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This article first appeared in the February, 2018 issue of Tech Briefs Magazine.

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